Spend a few minutes surfing Twitter and you’re likely to encounter a startling claim that comes without proof:

Cannabis cures cancer.

The online world is awash with such posts, startling scientists and physicians who are urging weed’s proselytizers to tap the brakes.

“We know that a component in cannabis — CBD — might be useful in treating cancer,” said Dr. Joseph A. Califano III, director of the Head and Neck Cancer Center at UC San Diego.

“But we don’t know if marijuana can stop or cure it. In some cases, cannabis might make things worse. It’s going to take time to find out.”

Califano added that, “What’s happening right now with marijuana reminds me of tobacco at the end of World War II. There was an explosion in its use, but little science to let people know what we were dealing with.”

The admonition appears to be falling on deaf ears.

Marijuana is increasingly described online as a magical elixir, not only for cancer but virtually every disease that afflicts humans. The claims are often unattributed and are rarely tied in a substantive way to mainstream science. And the proselytizing isn’t likely to fade.

Six months ago, it became legal in California to sell recreational marijuana in licensed pot shops. Customers have been lining up, and many turn to the store’s “bud tenders” to tell them which strain of weed can alleviate their aches and pains or help them to sleep.

Most bud tenders have no formal training in medicine or science.

Zach Lazarus recognizes the potential for abuse and works to avoid it.

“Our employees are not licensed physicians, nor do they pretend otherwise,” said Lazarus, co-founder of A Green Alternative, a legal pot store in Otay Mesa.

“They just help facilitate the sale of marijuana. The consumer uses his own discretion, and hopefully advice from his physician, if he needs it.”

The whole thing can be confusing for consumers, even if they turn to experts for advice. That’s led to many questions, some which are addressed here.

Q: One of the most shared marijuana claims on Twitter says, “Cannabis cures 7 out of 10 cancer patients.”

If that’s untrue, why are people allowed to say it?

A: The claim is untrue, and there are lots of reasons it gets out there, beginning with the First Amendment. There are few restrictions on what you can say or publish. And when a comment like that is posted online it can be difficult — maybe even impossible — to reign things in. Some of these posts are shared by bots, rather than humans, making things go faster. Scurrilous posts can go viral in a snap.

It also appears that a lot of people simply want such comments to be true. And that’s understandable, to a degree. About 610,000 people will die of cancer in the U.S. this year. People are desperate for good news.

The reality is that good news is hard to come by. There’s a website that claims that, “There Are Now 100 Scientific Studies That Prove Cannabis Cures Cancer.” The site links to scientific papers that focus on specific experiments. The papers do not provide clear, replicable evidence that marijuana can cure any form of cancer.

Many of the papers talk about experiments that were done on mice , which are used as a proxy for humans. As NPR noted in a story last year, “Mice aren’t simply furry little people.” Drugs and therapies that work on mice often don’t work on people. That’s why so many drug trails end in failure.

Q: Why is little known about whether marijuana is good or bad for your health?

A: The recreational use of marijuana only dates back about a century in the U.S. For a variety of reasons, the drug became associated with violence. It was soon banned in much of the country and drug laws became progressively tougher, pushing it further into the background.

Lots of people used weed during the counter culture movement. But that did not lead to widespread and well-funded studies that examined how cannabis affects human health. In fact, marijuana was classified as a Schedule 1 drug in the early 1970s as part of President Nixon’s so-called “War on Drugs.”

The DEA says that such drugs have “no currently accepted medical use and a high potential for abuse. Some examples of Schedule I drugs are: Heroin, LSD, marijuana, methaqualone, and peyote.”

The classification is opposed by much of the nation’s medical and scientific communities. But it’s still in place, which means that its hard for science to obtain federally approved marijuana, and funding, to conduct research.

Joy Phillips, a biomedical researcher San Diego State University, has received federal permission to conduct research on CBDs. But she doesn’t have large grants to pay for it, forcing her to consider online crowdfunding.

Q: Does this mean that scientists know basically nothing about the health implications of using marijuana?

A: No. There is evidence that cannabis can — to differing degrees — be used to treat a variety of medical issues, from back pain to seizures to the nausea associated with chemotherapy.

But the breadth and depth of the research isn’t wide or deep. That problem was underscored by the National Academics of Science, Engineering and Medicine, which released a 500-page overview of the health affects of cannabis and cannabinoids in January 2017.

The report noted that there were problems with many of the studies it examined, and that many health issues had been barely addressed. As a result, the report lacked specificity when it talked about such things as using marijuana to treat such things as multiple sclerosis, Tourette syndrome and heart disease.

The report’s main decision came down to this: Scientists need more money and reasonable access to federally approved cannabis to rectify the situation.